It is generally known that the 1-10 THz frequency region (also defined as far infrared) is difficult to reach with sources based on semiconductor devices or, more generally, solid-state devices (R. E. Miles et al., Terahertz Sources and Systems, NATO ASI Series, Kluwer 2001). In fact, electronic components based on the oscillation of free charges, such as Gunn diodes or resonant tunnel-effect diodes, can reach frequencies of about one hundred GHz at most. At the other end of the spectrum, conventional diode lasers operating on optical transitions from the conduction band to the valence band of the semiconductor material are typically limited to visible or near/middle infrared frequencies (>30 THz).
There is, however, very great technological interest in this region of the spectrum in view of the many requirements in the fields of spectroscopy, of wireless communications, and of the production of images for medical purposes or security checks. In fact, the particular transparency or opacity characteristics of various substances within this frequency range are very suitable for the examination of biological tissues (in a manner similar and complementary to X-rays) or for use in surveillance operations in which it is necessary to examine objects that are concealed from view by garments or plastics containers. Finally, the transparency of construction materials and the large bandwidth available make these frequencies an optimal choice for intra-building communications of the future.
In principle, the quantum-cascade lasers (QCLs) that have recently been developed offer the capability to generate electromagnetic radiation in the far infrared range. These are, in fact, unipolar devices operating on transitions between sub-bands of states belonging to the same conduction band, resulting from the quantum confinement of the electrons in a substantially two-dimensional heterostructure (J. Faist et al., Science 264, 553, 1994). The energy separation between these sub-bands, and hence the frequency of the photons emitted, therefore depends mainly on the thickness of the semiconductor layers in which the electrons are confined and not on the electronic structure of the original material. In the current state of the art, QCLs have been produced to cover the entire middle infrared range up to a maximum wavelength of 24 μm (12.5 THz) (R. Colombelli et al., Appl. Phys. Lett. 78, 2620, 2001). However, the production of a QCL operating in the THz range has remained impracticable up to now for various reasons. In the first place, there is the need to develop wave-guides of thicknesses (about 10 μm) compatible with the QCL growth system (molecular-beam epitaxy or MBE), which can effectively confine radiation of much longer wavelengths (˜100 μm) without increasing optical losses to prohibitive values. In the second place, there is the need to design the active region in a manner such as to ensure the population inversion that is necessary to compensate for the cavity losses. This latter need is more complex than in conventional QCLs owing to the fact that the energies involved become less than that of the optical phonon. This completely changes the dynamics of the non-radiative relaxation processes and requires a different approach on which to base the creation of the electronic structure.
In the current state of the art, there are therefore only QC devices that are capable of spontaneous emission at the frequencies of interest herein (with powers of the order of tens of pW in the THz range), without any evidence of laser effect or, even less, of gain (M. Rochat et al., Appl. Phys. Lett. 73, 3724, 1998 and J. Ulrich et al., Appl. Phys. Lett. 76, 19, 2000).
The present device, like other semiconductor lasers, is composed of an active material in which the electromagnetic radiation is generated by virtue of electron injection. This is introduced into a wave-guide which is capable of confining the radiation in the particular region of space which is occupied by the active material and which defines the lateral dimensions of the optical cavity that is necessary for the operation of the laser. Given the two-dimensional characteristic structure of the active regions of QCLs, it is necessary to implement a planar wave-guide which provides for the confinement of the radiation in the direction in which the semiconductor material is grown, leaving the definition of the cavity in the perpendicular directions simply to the processes by which the device is produced (lithography, etc.). At visible or near and middle infrared frequencies, this wave-guide is generally produced by enclosing the active material between two or more layers of a different semiconductor with a lower refractive index. As is known, by virtue of the principle of total internal reflection, a wave-guide generally called a dielectric wave-guide, with operation similar to optical fibres, is thus produced. However, this approach cannot be used for frequencies in the THz range (wavelengths of about 100 μm) since it would require thicknesses of the semiconductor layers of the order of or greater than the wavelength, which are absolutely impracticable for the growth techniques (MBE, MOCVD) that are generally used. Moreover, since injection devices are involved, the semiconductors used must have a predetermined level of doping to ensure optimal transport properties. This would result into very high losses by absorption since the absorption coefficient “k” of the free carriers in a doped semiconductor is proportional to the square of the wavelength and thus becomes enormous in the far infrared range (P. Y. Yu and M. Cardona, Fundamentals of Semiconductors, Springer-Verlag, Berlin, 1996). Recently, owing to the development of QCLs with wavelengths greater than 15 μm, a new wave-guide based on surface plasmons, has been used (C. Sirtori, et al., Opt. Lett. 23, 1366, 1998; A. Tredicucci et al., Appl. Phys. Lett. 76, 2164, 2000). Surface plasmons are optical modes that are confined at the interface between two materials with dielectric constants of opposite sign such as, for example, a metal and a semiconductor. They are TM-polarized (and are therefore very suitable for QC lasers which emit TM-polarized light) and have an electric-field profile with the maximum at the interface and an exponential decay on both sides in the direction perpendicular to the surface. If ε1 is the dielectric constant of the metal and ε2 is that of the semiconductor, the penetration of the surface plasmon into the two materials is given by:
                              δ                      1            ,            2                          =                              λ            2                    ⁢                                                                  Re                ⁡                                  [                                                            ɛ                                              1                        ,                        2                                                              ⁢                                                                                            -                          1                                                                                                      ɛ                            1                                                    +                                                      ɛ                            2                                                                                                                                ]                                                                                  -              1                                                          (        1        )            
The penetration into the metal layer will thus be less the more negative is its dielectric constant [Re(ε)=n2−k2]. This aspect is important because the metal is notably absorbent (k>>1) and too pronounced a penetration of the optical mode would cause unacceptable losses. This explains why wave-guides based on surface plasmons are effective only for lasers of sufficiently long wavelength (λ>15 μm), in which the dielectric constants of the metals become ever more negative (k2>>n2).
The order of magnitude of the loss in surface plasmon wave-guides formed in QC lasers of longer wavelength is about one hundred cm−1 (A. Tredicucci et al., Appl. Phys. Lett. 76, 2164, 2000; R. Colombelli et al., Appl. Phys. Lett. 78, 2620, 2001). Moreover, from the formula given above, since the dielectric constant of the semiconductor is relatively small and almost exactly real, it also seems clear that the penetration into the semiconductor is approximately inversely proportional to that into the metal (and in the far infrared range may thus also become very considerable). These characteristics mean that a surface plasmon wave-guide of the type used up to now is also not usable with success for a THz laser.